A disk drive is a digital data storage device that stores information within concentric tracks on a storage disk. The storage disk is coated on one or both of its primary surfaces with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. During operation of a disk drive, the disk is rotated about a central axis at a constant rate. To read data from or write data to the disk, a magnetic transducer (or head) is positioned above (or below) a desired track of the disk while the disk is spinning.
Writing is performed by delivering a polarity-switching write current signal to the transducer while the transducer is positioned above (or below) the desired track. The write signal creates a variable magnetic field at a gap portion of the transducer that induces magnetically polarized transitions into the desired track. The magnetically polarized transitions are representative of the data being stored.
Reading is performed by sensing the magnetically polarized transitions on a track with the transducer. As the disk spins below (or above) the transducer, the magnetically polarized transitions on the track induce a varying magnetic field into the transducer. The transducer converts the varying magnetic field into a read signal that is delivered to a preamplifier and then to a read channel for appropriate processing. The read channel converts the read signal into a digital signal that is processed and then provided by a controller to a host computer system.
When data is to be written to or read from the disk, the transducer must be moved radially relative to the disk. In a seek mode, the transducer is moved radially inwardly or outwardly to arrange the transducer above a desired track. In an on-track mode, the transducer reads data from or writes data to the desired track. The tracks are typically not completely circular. Accordingly, in the on-track mode the transducer must be moved radially inwardly and outwardly to ensure that the transducer is in a proper position relative to the desired track. The movement of the transducer in on-track mode is referred to as track following.
Many modern hard disk drives employ a dual-actuator system for moving the transducer radially relative to the disk. A first stage of a dual-actuator system is optimized for moving the transducer relatively large distances as is required during seek mode. A second stage of a dual-actuator system is optimized for moving the transducer relatively small distances to improve track following during on-track mode. The present invention relates to hard disk drives having dual-stage control systems.
FIG. 1 illustrates the major components of a standard disk drive, generally designated 10. FIG. 1 shows that the disk drive 10 includes a disk 12 that is rotated by a spin motor 14. The spin motor 14 is mounted to a base plate 16. The disk drive 10 includes at least one and typically a plurality of disks 12, each with one or two recording surfaces. The disk drive 10 further comprises control electronics generally designated by reference character 18 in FIG. 1. The control electronics 18 typically include a preamplifier, a read channel, a servo control unit, a microprocessor-based controller, and a random access memory (RAM). The control electronics 18 are or may be conventional and will not be described herein beyond what is necessary for a complete understanding of the present invention.
FIGS. 1 and 2 illustrate a mechanical portion 20 of a dual-stage control system. The mechanical portion 20 comprises a bearing assembly 22 that supports at least one actuator arm assembly 24. The actuator arm assembly 24 supports a head 26 adjacent to one recording surface 28 of one of the disks 12. The head 26 comprises the transducer described above; the head 26 will thus be referred to herein as the component that reads data from and writes data to the disk. Typically, the bearing assembly 22 will support one actuator arm assembly 24 adjacent to each of the recording surfaces 28 of each of the disks 12.
In the disk drive 10 having a dual-stage control system, each actuator arm assembly 24 includes a first arm 30 and a second arm 32. The bearing assembly 22 supports a proximal end 40 of the first arm 30 such that the first arm 30 rotates about a bearing axis A. A distal end 42 of the first arm 30 in turn supports a proximal end 44 of the second arm 32 such that the second arm 32 rotates about a second axis B. A distal end 46 of the second arm 32 supports the head 26.
FIG. 2 further illustrates that the hard drive 10 further comprises a primary actuator 50 and a secondary actuator 52. In many hard disk drive systems, the primary actuator 50 is formed by voice coil motor (VCM), while the secondary actuator 52 is formed by a piezo-electric transducer (PZT). In this application, the primary actuator 50 will be referred to as the VCM 50 and the secondary actuator 52 will be referred to as the PZT 52. However, other types of actuators may be used instead of a VCM and/or PZT. For example, the secondary actuator 52 may be formed by a micro-electromechanical system (MEMS) actuator.
The VCM 50 is coupled to the proximal end 40 of the first arm 30 such that operation of the VCM 50 causes the first arm 30 to move within a predefined range of movement about the bearing axis A. The PZT 52 is supported by the distal end 42 of the first arm 30 and is coupled to the proximal end 44 of the second arm 32; operation of the PZT 52 causes the second arm 32 to move through a predefined range of movement relative to the second axis B. Depending upon the specific implementation of the secondary actuator 52, this movement can be linear or can be rotational as depicted in FIG. 2.
FIG. 3 contains a block diagram of a control system 60 representing a conventional two-stage actuator system. The control system 60 comprises a first stage 62 and a second stage 64. As generally described above, the disk 12 defines a plurality of tracks 66 in the form of generally concentric circles centered about a spindle axis C. The first stage 62 controls the VCM 50 and the second stage 64 controls the PZT 52 to support the head 26 adjacent to a desired one of the tracks 66.
More specifically, an input signal “R” is combined with a position error signal “PES” by a first summer circuit 70. The second stage 64 generates a second stage position signal Y2 and a second stage position estimate signal “Y2est” based on the output of the first summer circuit 70. The second summer circuit 72 combines the second stage position estimate signal “Y2est” and the output of the first summer circuit 70. The first stage 62 generates a first stage position signal “Y1” based on the output of the second summer circuit 72. A third summer circuit 74 combines the first and second stage position signals “Y1” and “Y2”. System disturbances “d” are represented as an input to the third summer circuit 74. The position error signal “PES” is thus the result of combining the first and second position signals “Y1” and “Y2” with any system disturbances “d”.
The source of the input signal “R” is or may be conventional and will be described here only to the extent necessary for a complete understanding of the present invention. Each of the tracks 66 contains data sectors containing stored data and servo sectors containing servo data. The servo data identifies each individual track 66 to assist in seek operations and is also configured to allow adjustment of the radial position of the head 26 during track following. The source of the input signal “R” is thus generated by a servo control unit of the control electronics 18 based on the servo data read from the disk 12.
Referring now back to FIG. 2, that figure shows that the first arm 30 defines a first arm axis D and the second arm 32 defines a second arm axis E. The PZT 52 moves the second arm 32 relative to the first arm 30 such that the position of the second arm axis E varies relative to the first arm axis D. In particular, as shown in FIG. 2, a range of movement “S” associated with the secondary actuator 52 is defined by the stroke “s+” and “s−” in either direction relative to a neutral position defined by the first arm axis D.
When the disk drive 10 is in on-track mode, the secondary actuator 52 moves the head 26 relative to the first arm 30 as necessary to follow the desired track. The term “actual displacement” (ds in FIG. 2) refers to the difference at any point in time of the head 26 relative the neutral position as defined by the first arm axis D. When the head 26 is in the neutral position, the actual displacement is zero.
FIG. 2 further identifies arbitrary first and second tracks 66a and 66b on the disk 12. The actuator arm assembly 24 is shown in an initial position by solid lines and in a target position by broken lines; the first track 66a will thus be referred to as the “initial track” and the second track 66b will be referred to as the “target track”. It should be understood that the terms “initial track” and “target track” are relative to the position of the head 26 before and after a seek operation. Any track 66 on the disk 12 may be considered the initial track or the target track depending upon the state of the disk drive 10 before and after a particular seek operation.
FIG. 2 generally represents what will be referred to as a seek/settle process. The seek/settle process begins with the disk drive 10 originally in on-track mode with the head 26 following the initial track 66a. The disk drive 10 is then placed in seek mode to move the head 26 from the initial track 66a to the target track 66b. After the seek/settle process is completed, the disk drive 10 is then placed back into on-track mode with the head 26 following the target track 66b. No user data can be read from or written to the disk 12 for a substantial portion of the seek/settle process. Accordingly, when the disk drive 10 is used to store data as part of a larger system such as a personal computer, overall disk drive performance can be significantly affected by the characteristics of the seek/settle process.
One factor that contributes significantly to the duration of the seek/settle process is the amplitude of the off-track during settle. The secondary actuator has a potential to correct this off-track and thereby to improve the seek/settle performance. However due to the limited stroke of the secondary actuator its correction capability will be affected by the actual displacement. The term “initial displacement offset” will be used herein to refer to the actual displacement that exists when the drive 10 first enters seek mode. The term “displacement offset” will be used to refer to any residual actual displacement that exists because the initial displacement offset was non-zero when the disk drive 10 entered seek mode.
Ideally, no displacement offset will exist (first and second arm axes D and E aligned) at the start of any given seek operation. In this optimal case, the full range of movement of the second arm 32 is available to the second stage 64 for locating and following the target track 66b. In the worst case, the second arm 32 is at either extreme of its stroke. In this worst case, the second stage 64 can easily become saturated and non-responsive during a settle operation.
Typically, the displacement offset will be somewhere between neutral and the positive or negative maximum values. However, even a displacement offset value that is less than the maximum value can cause the second stage 64 to oscillate or “ring” as the dual-stage control system attempts to place the head 26 in a desired position relative to the target track. As long as the second stage 64 is saturated or ringing, the second stage 64 may be non-responsive and unable to cause the head 26 to track the target track 66.
Conventionally, several techniques have been used or proposed to accommodate displacement offset at the start of the seek/settle process. A first technique is simply to place the control system 60 in single-loop mode prior to initiating a seek operation. When the control system 60 enters single-loop mode, the second stage 64 is disabled, thereby “freezing” the displacement offset. The first stage 62 then moves the head 26 to the desired position relative to the target track, at which point the control system 60 is placed back in dual-loop mode to enable the second stage 64 and begin track following.
This first technique can cause several problems. First, when the control system 60 is changed from single-loop mode back to dual-loop mode, the value of the position estimate signal “Y2est” is equal to the displacement offset prior to entering single-loop mode. Depending on the value of the displacement offset upon entering single-loop mode, the position estimate signal “Y2est” may be the equivalent of injecting a significant step disturbance in the first stage loop when the system returns to dual-loop mode. This step disturbance can result in track misregistration that will increase settle time.
FIG. 4 illustrates the track misregistration that can occur when the value of the displacement offset is frozen upon entry into single-loop mode. FIG. 4 also compares the overall dual-stage performance with the performance of a high bandwidth single stage system. As depicted in FIG. 4, the settle associated with the two-stage actuator system is worse than that of the single stage system.
A second problem with the first technique is possible saturation of the second stage 64 over time. Freezing the displacement offset when placing the control system 60 in single-loop mode can define a false neutral position that may be offset from true neutral. After the hard drive 10 performs numerous seek/settle processes, the false neutral positions can accumulate such that the false neutral position is at or close to either the positive or negative maximum displacement value. As a result, the second stage 64 may immediately saturate and become non-responsive when the control system 60 is placed back in dual-loop mode.
FIG. 5 illustrates the track misregistration that can occur when the second stage 64 saturates because a false neutral position has been established. FIG. 5 also compares the overall dual-stage performance with the performance of a high bandwidth single stage system under similar conditions. Again, the settle associated with the two-stage actuator system is worse than that of the single stage system.
A second technique for decreasing seek/settle time is to set the displacement offset to zero at the beginning of the seek/settle process. Instantaneously zeroing the second stage 64 is the equivalent of injecting into the servo loop a step disturbance with a magnitude of equal to that of the displacement offset. Such a step disturbance may cause ringing that will adversely impact settle time.
FIG. 6 illustrates the track misregistration that can occur when the value of the displacement offset is reset to zero upon entry into single-loop mode. FIG. 6 also compares the overall dual-stage performance with the performance of a high bandwidth single stage system under similar conditions. Again, the settle associated with the two-stage actuator system is worse than that of the single stage system.
Accordingly, a need exists for a disk drive with a dual-stage actuator that reduces the duration of the overall seek/settle process and, in particular, which reduces the settle time caused by displacement of the second stage prior to entering the seek/settle process.